Known building materials made from inorganic binders include concrete and mortar. Conventional concrete or mortar compositions are made from mixtures containing an inorganic binder, e.g., cement, an aggregate such as gravel or sand, water and, optionally, additives and/or admixes. The terms "concrete" and "mortar" herein refer to compositions that differ only by the maximum particle size of the aggregates used to make a composition. The term "mortar" refers to compositions made from aggregates having a maximum particle size of up to 4 mm. The term "concrete" refers to compositions made from coarser aggregates. In the present invention, no further distinction is made between the terms "concrete" and "mortar."
To improve the processing and useful properties of concrete and mortar compositions, e.g., an increase in the composition's strength or resistance to chemicals, plastics have been increasingly employed as concrete additives. An overview of the large number of different polymers which have been tested for modifying concrete is disclosed in H. Schorn, "Betone mit Kunststoffen" [Concretes with Plastics], Ernst & Sohn Verlag fur Architektur und technische Wissenschaften, Berlin 1991, p. 25.
Polyisocyanates and polyurethanes have also been proposed as possible additives for mixtures used to prepare concrete or mortar compositions. DE-A 1,924,468, for instance, mentions aromatic polyisocyanates such as diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI) or derivatives thereof. The patent also discloses biurets and urethanes of aliphatic hexamethylene diisocyanates (HDI) as suitable additives for cement mixtures. When added to a concrete composition, either individually or in combination with additional polymers that are reactive with isocyanate groups, in the manufacture of floor coverings, these additives are said to accelerate the hardening of the cement mixtures, thereby enabling the floor to be walked on earlier than it would ordinarily be walked on. The faster hardening of concrete compositions or mortar compositions after addition of small amounts of organic (preferably aromatic) polyisocyanates is also known from EP-A 23,579. In both of these references the hydrophobic polyisocyanates are completely incompatible with water and cannot be stirred homogeneously into an inorganic binder, even if a considerable amount of an organic solvent is used.
However, in order to form a uniform polymer skeleton within the inorganic binder matrix (thereby resulting in a concrete or mortar composition having optimum final properties), the polymeric additive should be distributed in the binder mixture as uniformly as possible.
EP-A 181,261 describes the use of polyisocyanates in the form of finely divided aqueous dispersions for endogenous carbonatization of concrete. Hydrophobic MDI is exclusively employed here as the polyisocyanate, and it is premixed with water at an MDI:water ratio of 4:1 to 2:1 with the aid of a high-speed stirrer, i.e., it is added to the concrete composition in the form of a water-in-oil emulsion. Although such emulsions of water in MDI which are obtainable using high shear forces have in themselves a limited stability, when these emulsions are stirred into an excess amount of an aqueous system, as a concrete mixture is, no finely divided oil-in-water emulsions are obtained. Instead, instant phase separation occurs. Polyisocyanates also cannot be incorporated sufficiently homogeneously into concrete compositions or mortar compositions by the process of EP-A181,261.
According to the teachings of DE-A 2,300,206, the compatibility of hydrophobic polyisocyanates with cement compositions can be improved significantly when they are used in combination with water-soluble polymers such as cellulose derivatives, polyvinyl alcohols or polyether-alcohols (which optionally also contain additional emulsifiers.) However, even if organic solvents are used, the aqueous cement mixtures obtainable by the process described have only very short processing times of a few minutes, times which are not useful in practice. Furthermore, the considerable amount of water-soluble polymers incorporated into the binder in this manner lead to a permanent hydrophilicity and therefore lead to a concrete having undesirably high water absoption.
U.S. Pat. No. 4,143,014 describes another very specific process for incorporating a hydrophobic polyisocyanate into an aqueous system. According to this process, mixtures of water-soluble polyether-diols with excess equivalent amounts of MDI are stirred in water within a short period of time after the start but still before the end of the urethanization reaction to give clear stable solutions. According to U.S. Pat. No. 4,228,053, such solutions can be suitable for improving the stability properties of concrete. The precise point in time at which the reaction mixtures of MDI and polyether-alcohol have an adequate water-miscibility that lasts for a few minutes depends on the nature of the particular polyether-diol employed and can be determined only in involved, complicated, time-consuming preliminary experiments. This process therefore also has not found acceptance in practice.
All the processes mentioned above require either the use of additional organic compounds such as solvents, specific water-soluble polymers, or specific mixing units, i.e., high-speed stirrers in order to render the hydrophobic polyisocyanate component mixable with the inorganic binder even to a minor degree.
To overcome these disadvantages, attempts have also been made to employ, self-dispersible polyisocyanates which have been modified hydrophilically by incorporation of ionic groups (DE-A 2,359,611) as additives to inorganic binders. Although such polyisocyanates containing salt groups can be stirred into aqueous systems in very finely divided form without high shear forces, they have a completely inadequate storage stability in bulk. Because of the known catalytic activity of ionic groups, polymerization of the isocyanate groups occurs at room temperature, e.g., by the trimerization to polyisocyanurates or the formation of nylon structures. Such polymerization generally leads to gelling of the product after a few days.
It is an object of the invention to develop a new method for improving the processing and properties of concrete or mortar compositions without the disadvantages described in the prior art. It would be desired that the additives used in the method be storage-stable and free from organic solvents. It would further be desired that the additives be readily dispersible, in finely divided form, without the use of high-speed stirrers, and that the additives do not adversely influence the processing time of concrete compositions.
It has now been possible to achieve these objects by the use of specific polyether-urethanes as additives for inorganic binders in mixtures to make mortar or concrete compositions. Suitable polyether-urethane compositions are known per se and are described as surface-active agents or emulsifiers for hydrophobic polymers (poly-isocyanates in a number of publications, e.g., DE-A 2,415,435, EP-A 13,112, EP-A 19,844, EP-A 61,628, EP-A 110,497, EP-A 206,059, EP-A 281,801, EP-A 486,881, WO 96/30425).
The invention is based on the surprising observation that the addition of very small amounts of the water-dispersible or water-soluble polyether-urethanes to hydraulic binders in the preparation of mortar or concrete compositions considerably improves the mechanical properties of the concrete or mortar compositions. These properties include the compressive strength, the tensile strength, and the elasticity of the mortar or concrete composition. The polyether-urethanes can optionally contain free isocyanate groups.